imap: Work in progress of connection setup.

* SelectCommand now correctly encodes the mailbox name again.
* Hierarchy separators from the LIST/LSUB commands are now properly parsed,
  and taken into account.
* Folders in the file system are now created for mailboxes on the server,
  supporting hierarchical mailbox names.
* A connection worker now has an IMAPFolder that handles the local side of the
  mailbox, and an IMAPMailbox that handles the server side.
* Connection workers are now created/deleted, and setup correctly. They will
  now also wait in case they don't have a mailbox until they get some.